2 Answers
2

You are looking for the bacteria which have the gene (probably a plasmid) you (or someone else for you) put into the bacteria. That gene lets them live in the presence of kanamycin, and any bacteria that don't have it, or that try to get rid of it, will die. Thus you can select for the bacteria you want by killing everything else off (hopefully your sample is not contaminated with other bacteria also resistant to kanamycin).

I totally thought there would be a good wiki article on positive selection but there isn't. The reason it's positive selection is that you are selecting for bacteria that have something; instead of negative selection where you would be killing them for having something.

You can find plenty more if you search "positive selection" or more generally "artificial selection."

According to this data sheet, the genotype of the strain RosettaTM(DE3)pLysS is F- ompT hsdS (r- m-) gal dcm (DE3) pLysSRARE (CamR).

This strain carries certain tRNA genes on the same plasmid as the LysS gene, and these tRNAs boost expression from genes containing rare (in E. coli) codons. The plasmid encodes chloramphenicol resistance (CamR).

If you have a protocol that suggests growing your strain under kanamycin selection then presumably you have an additional plasmid present which carries your gene of interest. Then, as the accepted answer says, you are selecting positively for the correct strain.

I don't know how stable the pLysSRARE plasmid is, but I would have expected that you should also maintain selection for chloramphenicol resistance.